FIG. 1 shows a prior art power circuit 10 configured for remote sensing of a load voltage. Circuit 10 generally includes a power supply 12 configured to output a voltage to a load 14 via wires 16, 18. The power supply is regulated so as to output a stable output voltage, for instance 24 VDC. However, wires 16 and 18 inherently possess a certain resistance when current flows through the wires. By way of example, standard 24 gauge wire has a resistance of 2.567 ohms per 100 feet. This resistance causes a drop in voltage between the power supply and the load. Thus, if wires 16 and 18 are long, they may cause a significant voltage drop over the length of the run. To continue the above example, if wires 16 and 18 are 24 gauge wire having lengths of 350 feet, each wire would have a resistance of about 9 ohms equating to a total wire resistance of 18 ohms. If the load current is 220 mA, the calculated voltage drop across the load would be about 4 V (Vdrop=0.220 A*18 ohms). Thus, the load only receives 20 volts from the power supply. If the load requires 24 V for proper operation, the output voltage will need to be adjusted upwards about 4 V so as to output about 28 V to account for the voltage drop of the wires so that the required 24 V load voltage may be supplied to the load.
As shown in FIG. 1, prior art power supplies employ a pair of sense wires 20, 22 to provide remote sensing of the load voltage. Sense wires 20, 22 are selected so as to require minimal current and therefore result in low voltage drop across the sense wires. Sense wires 20, 22 operate to create a voltage feedback so that the power supply can adjust the output voltage until the sense wires sense the proper load voltage being supplied to the remote load. Sense wires 20, 22 may be shielded 26. While providing the necessary voltage regulation of the power supply to produce the required load voltage, sense wires 20, 22 increase the cost and complexity of the circuit by requiring additional wires to be run and the provision of dedicated sense wire terminals at the power supply.
What is needed in the art is a device that remotely senses the output voltage being supplied to a load without requiring the use of dedicated remote sensing wires. This device may be of particular advantage over long wire runs as the elimination of the dedicated remote sensing wires saves costs associated with this additional material and also simplifies installation by eliminating the need to properly wire the remote sensing wires to the remote sense terminals on the power supply. Moreover, in the case of a retrofit installation there may only be one pair of wires run between the power supply and the load.
It is a principal object of the present invention to provide a device for measuring voltage across a remote load wherein a switch such as, for example, a mechanical switch or a relay, transistor, MOSFET, or other solid state device, is integrated within the power supply circuit such that, when the switch is closed the device measures the output voltage of the power supply and, when the switch is opened the device measures the output voltage across a capacitor connected in parallel with the load. These two measurements are used to determine the voltage drop between the power supply and load such that the power output voltage supplied by the power supply may be increased so as to provide the desired load voltage at the load. The device may also include a second voltage sensor which is configured to allow for quick changes to the output voltage at the power supply to accommodate for load changes when the switch is in the closed position.